Homocystinemias are inborn errors of methionine and folate metabolism and occur by the deficient activity of either cystathionine synthase, tetrahydrofolate (THF) methyltransferase or methylene THF reductase. A new class of homocystinemias may be found due to other enzyme deficiencies in reactions associated with folate and methionine metabolism. The major obstacle in finding such milder forms of homocytinemias by the conventional methods has been the escape of free homocystine to a protein-bound form. Determination of protein-bound homocyst(e)ine (PBH) sensitively quantitates the level of homocystine in the plasma of normal subjects. Subsequently this new method has made it possible to discover two patients whose plasma PBH increased 10 fold greater than normal. No free homocystine was detectable from the plasma or urine in these patients. Clinically they are characterized either by recurrent episodes of pulmonary embolism or by coronary artery disease. Pedigree analyses indicate a possible association with a heritable trait. It appears that the biochemical and clincal characteristics of our patients do not fit under the documented system of homocystinemias. It is postulated that a deficiency of enzyme activity different from the currently documented enzymes is responsible for this protein-bound homocystinemia. Alternatively it may be hypothesized that a new mutation in any of the genes for the currently documented enzymes is the cause of PBH accumulation. In this project, these hypotheses will be tested by studying homocystine utilization inf fibroblasts obtained from the patients with an increased plasma level of PBH but no detectable free homocystine. We will be able to obtain a definitive answer through the identification of the enzyme responsible for this abnormality. For many years it was hypothesized that an elevated plasma level of homocystine is associated with the development of occlusive vascular disease. Thus the investigation of low degree homocystinemia in this proposal may provide new insights into the understanding of intravascular thromboembolism and arteriosclerosis.